Can the compiler deal with the initialization order of static variables correctly?

Question

How about the following case? Can the compiler deal with the initialization order of static variables correctly if there is dependency?

a.h

template<class T>
struct A { static double a; };

template<class T>
double A<T>::a = 1;

b.h

struct B { static double b; };

b.cpp

#include "b.h"
#include "a.h"

double B::b = A<int>::a;

Also: http://stackoverflow.com/questions/211237/static-variables-initialisation-order — anjruu, Sep 16 '13 at 18:34
In this case, there's no issue since `a` is initialised statically. Presumably, you're asking about the more general case where both require dynamic initialisation? — Mike Seymour, Sep 16 '13 at 18:46
@Mike Seymour: Isn't the initialization of `B::b` dynamic, since `A::a` is not a constant expression? — Vaughn Cato, Sep 16 '13 at 18:47
@VaughnCato: You're right; I should have said "no issue since `a` is initialised statically". — Mike Seymour, Sep 16 '13 at 18:48

Mike Seymour · Answer 1 · 2013-09-16T18:58:22.370

In this case, there is no issue since a is initialised statically; this is guaranteed to happen before the dynamic initialisation of b.

More generally, where both require dynamic initialisation, this is an issue. Unless you specialise the template member before using it to initialise b, there is no guaranteed order. To quote the standard (C++11 3.6.2/2):

Definitions of explicitly specialized class template static data members have ordered initialization. Other class template static data members (i.e., implicitly or explicitly instantiated specializations) have unordered initialization.

score 1 · Accepted Answer · answered Sep 16 '13 at 18:50

1

In your example, A<int>::a is initialized statically, and all static initialization takes place before any dynamic initialization. B::b is initialized dynamically and so it will happen after A<int>::a is initialized, and everything is fine.

answered Sep 16 '13 at 18:50

Vaughn Cato

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Can the compiler deal with the initialization order of static variables correctly?

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